Low voltage tube circuits

ABSTRACT

A number of low voltage vacuum tube circuits include using supply voltages well below the manufacturer&#39;s recommended voltages applied to the plate or screen grid. Some of the tube circuits operate at near zero plate and or screen grid voltages. Other low voltage circuits have forward biasing on one or more grids that are normally biased at a non positive voltage or a grid that is normally connected a cathode. Substantially lower supply voltages allow for example, the filament supply to also supply voltage to the plate and or grid for providing an output signal at a grid and or a plate.

This application claims priority to U.S. provisional Ser. No. 62/175,246filed on Jun. 13, 2015, and to U.S. provisional Ser. No. 62/175,285filed on Jun. 14, 2015, which both are incorporated herein by reference.

BACKGROUND

The present invention relates to operating vacuum tubes at lower thanthe normally specified plate or screen grid voltages.

Specially made tubes (e.g., including low voltage space charge tubesthat operates at about 12 volts on the plates and or screen grid orspace charge grid) such as those manufactured for automobiles in the1950s allowed for low plate voltages. However, conventionallymanufactured vacuum tubes require high voltages. The specification datasheets of these types of tubes generally require high voltages appliedto the plates and or screen grids.

SUMMARY

In one embodiment of the invention, it was found experimentally andunexpectedly that some vacuum tubes are able to output a signal withvery low plate or screen grid voltages. For example, a plate voltage wasprovided in a zero volt range (e.g., plate voltage between −1 volt and+1 volt, or a plate to cathode or filament voltage in the range of −1volt and +1 volt) in a conventional multi-grid tube or pentode or triodeprovided signal via a load element, such as a load resistor, inductor,transformer, active load, and or the like. Normally vacuum tubes thatare designed for high voltages (e.g., plate and or screen voltages atleast 50 volts) show signal output (or amplification) when operating atplate and or screen voltages as low as zero volts. For example thesignal input may be coupled to a first grid and the output via a loadresistor tied to a supply voltage that may include (e.g., around) zerovolt (e.g., may include a plate and or grid grounded and or tied to asupply voltage). The load element (e.g., resistor, network, circuit,impedance component) outputs a signal.

One or more embodiments may include the following:

1) A triode with a load resistor or element coupled to the plate and apower supply, wherein the power supply may provide a positive voltage ora voltage as low as zero volts or a negative voltage (e.g., platevoltage with respect to cathode or filament, or wherein the cathode iscoupled to ground, to an input signal, or to a positive voltage). Signalis coupled to a grid of the triode and an output signal is provided atthe plate.

2) A multiple grid tube is configured as a triode with a load resistoror element coupled to the plate or a second grid and to a power supply.Wherein the power supply may provide a positive voltage or a voltage aslow as zero volts or a negative voltage (e.g., plate voltage withrespect to cathode or filament, or wherein the cathode is coupled toground or to a positive voltage). Signal is coupled to a first grid (orto a cathode) of the multiple grid tube and an output signal is providedat the plate or the second grid.

3) A tetrode or pentode or multiple grid tube (e.g., a tube with two,three, four, five, six, or at least seven grids) with a load resistor orelement coupled to the plate and a power supply, wherein the powersupply may provide a positive voltage or a voltage as low as zero voltsor a negative voltage (e.g., plate voltage with respect to cathode orfilament, or wherein the cathode is coupled to ground or to a positivevoltage). Signal is coupled to a first grid (or to a cathode) of thetetrode or pentode or multiple grid tube and an output signal isprovided at the plate. A second grid or screen grid may be left open,coupled to the plate, coupled to a power supply, and or coupled to avoltage that is near or at ground or 0 volt. Near 0 volt may include apositive or negative voltage. Near 0 volt may include zero volt orsubstantially zero volt, or approximately zero volt.

4) A tetrode or pentode or multiple grid tube (e.g., a tube with two,three, four, five, six, or at least seven grids) with a load resistor orelement coupled to the plate or a second grid and a power supply,wherein the power supply may provide a positive voltage or a voltage aslow as zero volts or a negative voltage (e.g., plate voltage withrespect to cathode or filament, or wherein the cathode is coupled toground, a signal source, or to a positive voltage). Signal is coupled toa first grid (or to a cathode) of the tetrode or pentode or multiplegrid tube and an output signal is provided at the plate. The plate maybe left open, coupled to a power supply, and or coupled to a voltagethat is near or at ground or 0 volt. Near 0 volt may include a positiveor negative voltage. Near 0 volt may include zero volt or substantiallyzero volt, or approximately zero volt.

5) A tetrode or pentode or multiple grid tube (e.g., a tube with two,three, four, five, six, or at least seven grids) with a load resistor orelement coupled to the plate and a power supply, wherein the powersupply may provide a positive voltage or a voltage as low as zero voltsor a negative voltage (e.g., plate voltage with respect to cathode orfilament, or wherein the cathode is coupled to a ground, a signalsource, or to a positive voltage). Signal is coupled to a first grid (orto a cathode) of the tetrode or pentode or multiple grid tube and anoutput signal is provided at the plate. A second grid or screen grid maybe left open, coupled to the plate, coupled to a power supply, and orcoupled to a voltage that is near or at ground or 0 volt. Near 0 voltmay include a positive or negative voltage. Near 0 volt may include zerovolt or substantially zero volt, or approximately zero volt.

6) A tetrode or pentode or multiple grid tube (e.g., a tube with two,three, four, five, six, or at least seven grids) with a load resistor orelement coupled to the plate and a power supply, wherein the powersupply may provide a positive voltage or a voltage as low as zero voltsor a negative voltage (e.g., plate voltage with respect to cathode orfilament, or wherein the cathode is coupled to a ground, a signalsource, or to a positive or negative voltage). Signal is coupled to afirst grid (or to a cathode) of the tetrode or pentode or multiple gridtube and an output signal is provided at the plate. A second grid orscreen grid may be left open, coupled to the plate, coupled to a powersupply, and or coupled to a voltage that is near or at ground or 0 volt.Near 0 volt may include a positive or negative voltage. A power supplyvoltage is coupled to a second grid or a screen grid that results inhaving the output signal change phase. For example, when the powersupply voltage is at zero or near zero for the plate and screen gridsupplies, the output signal at the plate is 180 degrees phase shifted(e.g., as expected for a common cathode amplifier) from the input signalat a control grid. With the plate supply at zero or near zero volt, andthe screen grid or second grid voltage is increased in a positivedirection such as to around 4 volts (e.g., in a 12AC6 tube), theresulting output signal at the plate is substantially in phase with thecontrol grid signal, which is unexpected (e.g., when the cathode iscoupled to ground or a positive voltage).

For the previously stated 1 through 6, the plate does not need to becoupled to a plate or load resistor or element. Instead the platecurrent can be coupled to a load different from a resistor. For example,the load may be another amplifier, a transimpedance amplifier, or someother device (e.g., active load, inductor, element, transformer,capacitor, amplifier, circuit, and or solid state device). The platevoltage may be coupled or held to any of the plate voltages mentioned in1 through 6 (e.g., above). Note for the previously stated 1 through 6other voltage ranges may be included or used, and or other loadresistors and or elements may be included or used.

For the previously stated 1 through 6, the cathode may be coupled toground, a signal source, or to a positive voltage. For example, coupling(e.g., to the cathode) may include a resistor, inductor, capacitor,active current source, active load, amplifier, circuit, or a signalsource, or a voltage or current source.

For the previously stated 1 through 6, an output may be from a grid,plate, and or cathode.

An example summary of certain experiments (e.g., an amplifier or commoncathode amplifier) is shown in Tables A, B, or C below. Input voltage toa first grid or a control grid is an AC voltage at 0.200 volt peak topeak. Note that other AC voltages in terms of amplitude may be used thatare coupled to the first grid or the control grid.

The low voltage tube circuits or any embodiments may include use foramplifiers, mixers, multipliers, sound effects (e.g., musical soundeffects such as for guitar, fuzz, pedal, or electronic sound effects),adders, subtractors, and or feedback circuits. For example, one or moresignals may be coupled to a low voltage tube circuit at one or moreelements of the tube (e.g., signal coupled to a grid and or another gridand or cathode).

One embodiment may include enabling conventional high voltage tubes tooperate at lower voltages by (e.g., forward) biasing (e.g., via apositive voltage with respect to the cathode) a first grid closest tothe cathode (e.g., cathode or filament) and coupling signal to asubsequent grid such as a second or a third grid (e.g., a signal grid)that is further away from the first grid (e.g., wherein the cathode,heater, or filament is defined as a center reference). A load element iscoupled to a supply from 0 volts to a lower than normal (e.g., positive)voltage supply to a plate or to one or more grids beyond (or before) thesignal grid. Note that the voltage at the first grid may be a static ortime vary voltage. For example, a positive DC voltage into the firstgrid with respect to the cathode may be static or may be varied as toprovide a voltage controlled gain amplifier. In another example, thefirst grid's voltage may be time varying to provide a mixing,multiplying, or modulation effect on the signal coupled to thesubsequent grid.

Any embodiment may include directly or indirectly heated cathodes for avacuum tube. For example, a directly heated tube has a filament, or anindirectly heated cathode generally includes a filament and a sleeveencasing the filament, wherein the sleeved portion provides a cathodeconnection terminal.

Tables A, B, and or C show examples where low, zero, or close to zerovoltage provided to the plate or plate supply or grid supply will allowfor (e.g., usable) output signals.

TABLE A SECOND VOUT, OR PLATE OR OUTPUT SUPPLY SCREEN SIGNAL PLATEVOLTAGE GRID VOLTAGE LOAD OR V_(p) IN SUPPLY PEAK TO RESISTOR DISTORTIONTUBE VOLT VOLTAGE PEAK OR RL COMMENTS HD_(2,) HD₃ 6BA6 RCA**  47 V 0.00V  6.15 V 100 KΩ  6BA6 RCA**  6.2 V 0.00 V  2.17 V 100 KΩ  CLIPPING 6BA6RCA**  2.5 V 0.00 V  0.37 V 100 KΩ  CLIPPING 6BA6 RCA** 0.87 V 0.00 V0.061 V 100 KΩ  NO CLIPPING 6BA6 RCA** 0.00 V 0.00 V 0.006 V 100 KΩ  NOCLIPPING 6BA6 RCA** 0.00 V 0.00 V 0.019 V ~1 MΩ NO CLIPPING 6AU6** GE #10.00 V 0.00 V NO ~1 MΩ OUTPUT 6AU6** 0.00 V 0.00 V 0.046 V ~1 MΩCLIPPING HD₂ = 10%, MAGNAVOX HD₃ = 3.6% 6AU6** 0.00 V 0.00 V 0.004 V ~1MΩ DISTORTED 6AU6** GE #2 0.00 V 0.00 V 0.264 V ~1 MΩ HD₂ = 30%, HD₃ =7.0% 6AU6** GE #3 0.00 V 0.00 V 0.184 V ~1 MΩ HD₂ = 30%, HD₃ = 10.0%6BA6/5749 0.00 V 0.00 V 0.070 V ~1 MΩ HD₂ = 5%, GE** HD₃ = 0.30%6BA6/5749 0.00 V 2.00 V 0.131 V ~1 MΩ HD₂ = 3%, GE** HD₃ = 0.36% 6AU6**GE #2 0.00 V 0.00 V 0.360 V ~1 MΩ RETESTED HD₂ = 5%, WITH HD₃ = 3.0%SCREEN GRID TIED TO PLATE (E.G., TRIODE) * = low voltage tube **= highvoltage tube

Table B is below.

TABLE B SECOND VOUT, OR PLATE OR OUTPUT SUPPLY SCREEN SIGNAL VOLTAGEGRID VOLTAGE OR V_(p) IN SUPPLY PEAK TO LOAD DISTORTION TUBE VOLTVOLTAGE PEAK RESISTOR COMMENTS HD_(2,) HD₃ 6AU6** NOT 0.00 V 0.360 V ~1MΩ RETESTED HD₂ = 5%, GE #2 APPLICABLE, WITH LOAD HD₃ = 3.0% SEERESISTOR TO COMMENTS SCREEN GRID. PLATE IS DISCONNECTED AND IS OPENCIRCUIT AND FLOATING 6AU6** 0.00 V 0.00 V 0.544 V ~1 MΩ RETESTED GE #2WITH LOAD RESISTOR TO SCREEN GRID. PLATE IS DISCONNECTED AND ISCONNECTED TO GROUND (E.G., 0 VOLT) 6AU6**  47 V 0.00 V 0.590 V ~1 MΩRETESTED GE #2 WITH LOAD RESISTOR TO SCREEN GRID. PLATE IS DISCONNECTEDAND CONNECTED TO A PLATE SUPPLY (E.G., 47 VOLTS) * = low voltage tube**= high voltage tube

Table C is below.

TABLE C SECOND VOUT, OR PLATE OR OUTPUT SUPPLY SCREEN SIGNAL PLATEDISTORTION VOLTAGE GRID VOLTAGE LOAD HD_(2,) HD₃ OR OR V_(p) IN SUPPLYPEAK TO RESISTOR OTHER TUBE VOLT VOLTAGE PEAK OR RL COMMENTS COMMENTS6AU6** 0.00 V 3.16 V  0.230 V ~1 MΩ OUTPUT HD₂ = 2%, SYLVANIA SIGNAL HD₃= 3.6% PEAKS AT NOTE THAT ABOUT 3.16 THERE IS VOLTS DC MORE FOR SCREENTHIRD GRID OR ORDER SECOND DISTORTION GRID, OTHER THAN VOLTAGES SECONDABOVE OR ORDER. BELOW 3.16 UNUSUAL VOLTS FOR A RESULTS IN COMMON LOWERCATHODE OUTPUT AMPLIFIER. SIGNAL 12AC6* 0.00 V 0.00 V 0.0085 V ~1 MΩOUTPUT WESTINGHOUSE SIGNAL IS INVERTED AS EXPECTED FOR A COMMON CATHODEAMPLIFIER 12AC6* 0.00 V 4.12 V 0.0075 V ~1 MΩ OUTPUT ADJUSTINGWESTINGHOUSE, NOTE: THE SIGNAL IS THE INCREASE NON- SCREEN IN SCREENINVERTED GRID OR 2^(ND) OR 2^(ND) AND IS GRID GRID UNEXPECTED VOLTAGEVOLTAGE FORA FROM 0 CAUSED A COMMON TO +5 VOLTS PHASE CATHODE CHANGEDREVERSAL AMPLIFIER GAIN AND AT OR PHASE OUTPUT. AT OUTPUT. 12K5* RCA0.00 V 23.0 V 0.0045  ~1 MΩ HD₂ = 3% *= low voltage tube **= highvoltage tube

Note in Table C, tubes 12AC6 and 12K5 are special low voltage tubesdesigned to operate at 12 volts for the plates. Other tubes listed inTables A, B, and or C are conventional tubes designed to operate athigher voltages (e.g., 100 volts or more).

A 12DZ6 low voltage tube had similar results as a 12AC6 (e.g., as shownin Table C).

Certain (e.g., conventional, high voltage, and or low voltage) tubes dowork with (e.g., approximately, close to, or at) 0 volt plate and orscreen grid voltage.

Also, one or more tubes provide low distortion or higher distortionsignal output. The high distortion signals can be used forpredistortion, sound effects, and or musical distortion effects (e.g.,for a guitar, electric or acoustic, or for a musical instrument).

Of course, other conventional and or low voltage tubes can operate atclose to or at zero volts for the plate and or screen or second grid.

In a certain configuration using a pentode or a tube with 3 or moregrids, the third grid may or may not be grounded or coupled to a voltagesource. However, it (e.g., a third grid) can be connected in any manner.For example, such as connected to any of other the other grids or plate,connected to the cathode, connected as an open circuit, and or connectedto a voltage source. The voltage source can include a signal source, andor a voltage source including a negative or positive voltage or a zerovoltage.

Embodiments, circuits, and or experiments may include using single grid,multi-grid, or pentagrid or hexagrid tubes where the load resistor (orelement) or output current may be connected to any combination of one ormore grid(s) and or plate. Low supply voltages (e.g., positive ornegative or zero volt) may be connected to any combination of one ormore grid(s) and or plate.

Embodiments include the following:

A) Operating “high voltage” or conventional (e.g., non-low voltageautomobile tubes or specifically designed low voltage tubes), at lowerthan normal plate voltages (e.g., <100 volts)

B) Using a combination of high and low voltage tubes operating at lowplate voltages to provide amplification to provide a signal output.

C) Providing cascaded and or cascode circuits operating at low platevoltages.

D) Providing low noise performance for pre-amplification using low platevoltages.

E) Providing (e.g., line level or low level, or high level)amplification, or signal output using low plate voltages (e.g., withconventional tubes and or low voltage tubes).

F) Operating vacuum tubes at nearly zero volts (or zero volts) on theplate (e.g., ˜0 volts from plate to cathode while providing an AC signaloutput at the plate).

G) Using other grid(s) as output terminal(s) while operating at nearlyzero volts (or zero volts).

H) Forward biasing conventional high voltage tube(s) on a control gridto provide operating at a lower plate voltage for outputting an ACsignal.

I) Forward biasing conventional high voltage tube(s) on a control gridand one other grid to provide operating with a lower plate voltage foroutputting an AC signal.

J) Forward biasing conventional high voltage tube(s) on a control gridor a second grid to provide operating at a lower plate or third gridvoltage for outputting an AC signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a common cathode circuit.

FIG. 2 shows a high voltage feedback amplifier.

FIG. 3 shows another high voltage feedback amplifier.

FIGS. 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13 show examples of low voltagetube circuits.

FIG. 14 shows a multiple cascoded circuit.

FIG. 15 shows a feedback cascode circuit using an amplifier.

FIG. 16 shows an example transistor amplifier.

FIG. 17 shows an example FET (e.g., Field Effect Transistor) amplifier.

FIG. 18 shows an example tube amplifier.

FIG. 19 shows an example FET amplifier example.

FIG. 20 shows another transistor amplifier example.

FIG. 21A shows an example a very low voltage tube circuit.

FIG. 21B shows another example low voltage tube circuit

FIG. 22A shows an example of a low voltage tube circuit.

FIG. 22B show another example of a low voltage tube circuit.

FIGS. 23A, 23B, 23C, and 23D show alternate examples of low to very lowvoltage tube circuits.

DETAILED DESCRIPTION

In the following example circuits including FIGS. 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21A, 21B, 22A, 22B,23A, 23B, 23C, and or 23D other values for resistors, capacitors, and orvoltages may be used and or other type tubes may be used. For Tables A,B, C, 1, 2, 3, 4, 5, 6, 7, and or 8 other tubes, component values,voltages, and or currents (e.g., plate current and or grid current) maybe used. One or more embodiment(s) provide at least one improvement totube amplifiers.

Note that Fig.=Figure. That is, the abbreviation, “Fig.”, isinterchangeable with the word, “Figure” (e.g., when referring todrawings). Also TABLE=Table.

When it comes to maximum gain and widest voltage swings for a givenpower supply, pentodes have distinct advantages over triodes.

For example, a low voltage triode such as the 12AE6, which looks like a12AV6 tube, has a mu of only 15. This tube has a combination of lowtransconductance at 1 mmho and low plate resistance of 15KΩ.

In contrast, a 12EZ6 low voltage pentode has a transconductance of 2.7mmho and a plate resistance of 400KΩ. The equivalent mu is thetransconductance, g_(m), multiplied by the plate resistance, R_(p).Thus, for a 12EZ6, g_(m)×R_(p)=2.7 mmho×400KΩ=1080=mu.

FIG. 1 shows a circuit with plate supply voltages, 12 volts to 45 voltsalong with screen grid voltages from 6 volts to 24 volts. The plate loadresistance is 1KΩ to allow for easy calculation of the transconductance,g_(m). Vin=0.1 volt peak to peak AC signal at 1 kHz (e.g., otherfrequencies may be used).

V_(p)=Supply voltage in volts (v)

I_(p)=Plate current in milliamps (mA)

V_(g2)=Grid 2 or screen grid voltage

Vout=Output AC signal voltage in volts peak to peak (p-p).

g_(m)=transconductance in milli-mhos (mmho) or mS. Note: 1 mmho=1 mS

HD₂=Second harmonic distortion in percent

HD₃=Third harmonic distortion in percent

Table 1 shows a summary of the tests.

TABLE 1 Summary of various pentodes V_(p) I_(p) V_(g2) Vout g_(m) HD₂HD₃ TUBE (v) (mA) (v) p-p (mS) (%) (%) 12AC6* 12 0.83 12 0.100 1.000.800 0.080 12AC6* 12 1.87 24 0.110 1.10 0.055 0.071 12AC6* 24 4.30 470.135 1.35 0.036 0.071 12AC6* 45 2.11 24 0.145 1.45 0.360 0.030 12AF6*24 3.4 24 0.256 2.56 0.600 0.030 12AF6* 24 6.74 47 0.223 2.23 0.8000.030 12AF6* 45 3.5 24 0.266 2.66 0.800 0.030 12AU6** 45 0.50 24 0.1521.52 2.700 0.071 12AU6** 45 1.5 47 0.254 2.54 0.700 0.080 12BA6** 450.25 6.0 0.078 0.78 3.600 0.015 12BA6** 45 0.50 10.6 0.116 1.16 3.0000.024 12BA6** 45 1.60 24 0.226 2.26 1.500 0.050 12BA6** 45 3.77 47 0.2962.96 0.800 0.050 12DZ6* 45 8.67 24 0.530 5.30 0.800 0.036 12EK6* 45 7.2724 0.474 4.74 0.450 0.019 12EZ6* 45 4.87 24 0.442 4.42 1.000 <0.015 *=low voltage tube **= high voltage tube

A way to provide more voltage gain out of tubes operating at lowvoltages is to “starve” the plate current such that the transconductancehas not dropped too much (e.g., which allows for using a higher valueplate load element, resistor, or impedance device). This leads toanother experiment, circuit, or embodiment with high voltage pentodes,the plate load resistor, R3 is changed from 1KΩ to 100KΩ. Vin=0.1 voltpeak to peak AC signal. See Table 2.

TABLE 2 Selected pentodes with reduced plate current, which allows forhigher voltage gain. V_(p) I_(p) V_(g2) Vout g_(m) HD₂ HD₃ TUBE (v) (mA)(v) p-p (mS) (%) (%) 6BJ6** 45 0.13 6.0 5.060 0.506 2.000 0.150 12BA6**45 0.25 6.0 5.500 0.550 0.300 0.100 * = low voltage tube **= highvoltage tube

A method of starving the screen grid voltage to allow for highresistance loads, high gains can be achieved. For example, we willreiterate the following information from Table 2 where the plate loadresistor is 100KΩ. Again, Vin=0.1 volt peak to peak AC signal. See Table3. Note: g_(meq) is the equivalent transconductance based on themeasured voltage gain.

TABLE 3 Voltage gain of two vacuum tube amplifiers. V_(p) I_(p) V_(g2)Vout g_(meq) TUBE (v) (mA) (v) p-p (mS) GAIN 6BJ6** 45 0.13 6.0 5.0600.506 50.6 12BA6** 45 0.25 6.0 5.500 0.550 55.0 **= high voltage tube

In another experiment, circuit, or embodiment with the 6BJ6, plate loadresistor, R3, is raised from (e.g., about) 100KΩ to 200KΩ. The resultingAC output signal increased from 5.060 volts peak to peak to 9.045 voltspeak to peak. Had the tube provided infinite internal plate to cathoderesistance we would have expected twice of 5.06 volts peak to peak orabout 10 volts peak to peak. Instead, the voltage output voltage isslightly smaller at 9.045 volts peak to peak. From about a 10% loss fromthe ideal situation, the inferred internal plate resistance is about 2MΩfor 0.13 mA plate current. Nevertheless, the voltage gain with a 200KΩplate load resistor is about 90.45. Note that the voltage drop acrossthe 200KΩ with 0.13 mA flowing through it is 26 volts, leaving 19 voltsDC at the plate.

Similarly this plate current starving method works for the 12BA6 whosegain magnitude, |Vout/Vin|=5.500/0.100=55.0 for a 100KΩ plate loadresistor. Although the plate current is higher at 0.25 mA, the voltagedrop across the 100KΩ resistor is 25 volts, which leads to 20 volts atthe plate. Note that the screen grid voltage shown in TABLE 2 is 6volts, which is very low for high voltage or conventional tubes. In datasheets for both of these tubes (e.g., RCA Receiving Tube Manual RC-29,published in 1973), the recommended screen grid voltages are 100 voltsfor the 6BJ6 and 12BA6 with a 100 volt plate supply. Operating thescreen grid voltages at 6 volts or at a much lower voltage than therecommended values is unexpected when for example voltage gains areprovided.

FIG. 2 shows a prior art high voltage magnetic cartridge phonopreamplifier that operates at 350 volts. This example high voltagepreamp uses two common cathode amplifier stages.

FIG. 3 shows another prior art high voltage magnetic cartridge phonopreamplifier that operates at 350 volts. This example high voltagepreamp uses two common cathode amplifier stages with an added cathodefollower stage to allow for increased open loop gain and extra currentdriving capability into its own (e.g., RIAA phono equalization) feedbacknetwork (e.g., including R6, C6, R9, C7). In both FIG. 2 and FIG. 3,high voltages are required such as 350 volts. In prior art similar phonopreamplifiers, the plate supply usually exceeds 100 volts.

FIG. 4 shows a preamplifier design with low screen grid voltages such as8.6 volts. The preamp operates at less than 50 volts for the platesupply. Although the screen grid voltage is at 8.6 volts in thisexample, other voltages such as voltages ≤50 volts and ≥0 volts may beused. In FIG. 4 the first two stages use conventional high voltagetubes, whereas the cathode follower stage uses a low voltage tube.Alternatively, the cathode follower stage may include a high voltagetube instead of a low voltage tube.

FIG. 5 shows another preamplifier design with slightly higher screengrid voltages at about 27 volts, which is well below the 100 volts asstated in the tube data sheet.

To achieve improved noise performance of a preamp, triodes are used inplace of pentodes. Note: A triode may include a multi-grid (e.g.,tetrode, pentode, hexode, heptode, octode, or the like) tube wherein oneor more grid(s) is coupled to a plate, or when a grid provides an outputsignal. Triodes generally have lower voltage gain than pentodes becausetheir plate resistances are generally lower than pentodes. To raise theplate resistance and or increase the voltage gain, a cascode amplifieris used and it shown in FIG. 6. In FIG. 6 the cascode amplifier includesa common cathode stage to receive an input signal at its control gridfor example in V1A. The plate output of V1A is then coupled to thecathode of V1B, whose grid is supplied by a voltage potential that is anAC (alternating current) ground. The plate of V1B is coupled to anoutput terminal (e.g., Vout).

FIG. 6 shows a cascode circuit that allows forcing more plate current(via voltage source +Vbias and resistor R3) into the bottom tube whilereducing the DC plate current for the top tube, V1B. By reducing the DCplate current in the grounded grid amplifier tube V1B, a higherresistance value plate load resistor can be used, which will allow forhigher AC signal gain. Fortunately, most of the signal current from theplate of the common cathode amplifier V1A will flow into the lowimpedance, 1/g_(mV1B), cathode terminal of V1B. Very little signalcurrent is diverted through R3 whose resistance is 47KΩ, which is muchgreater than 1/g_(mV1B).

There is also a cathode resistor, R2, which may be part of a feedbacknetwork. By applying an AC signal (e.g., 0.033 mV peak to peak), we willdetermine the gain at Vout. Various tubes will be tested to determinethe highest voltage gain.

It was found that varying +Vbias affected the cascode amplifier's gain.Table 4 shows maximum voltage gain based on the final adjusted +Vbias DCvoltage. Note: +Vbias may be varied to provide a voltage controlled(e.g., gain) amplifier. Note that +Vbias may include a positive, zero,or negative voltage.

TABLE 4 Gain of cascode amplifier from FIG. 6 TUBE +Vbias (v) |GAIN|6BQ7** 13.3 27.9 6DJ8** 14.0 40.5 6GM8* 32.4 36.4 6N2** 15.3 37.1 6N6**12.7 15.0 ECC99** 10.9 9.7 12AT7** 15.0 35.5 12AU7** 24.1 33.3 12AV7**13.8 38.2 12AY7** 13.9 33.9 12AX7** 10.0 18.5 12BH7** 29.8 38.2 12DT8**13.6 36.1 12FQ7** 18.0 40.9 12U7* 19.5 32.7 Vin = 0.033 volt peak topeak AC signal. *= low voltage tube **= high voltage tube

FIG. 7 shows a cathode follower circuit, which is used for testingvarious output voltage swings (e.g., maximum voltage swing for a givenplate supply voltage).

This circuit in FIG. 7 is different from the active current source(e.g., V4) 12DU7 cathode follower (e.g., V3) shown in FIG. 4 thatoutputted about 18 volts peak to peak. More output voltage swing with asimpler circuit was achieved by choosing the best triode for thiscircuit in FIG. 7. See Table 5. (Note that R6 or a resistor may bereplaced with a current source or circuit.)

TABLE 5 Output signal voltage from the FIG. 7 cathode follower circuit.TUBE Vout p-p 6DJ8** 15 6GM8* 22 12FQ7** 11 *= low voltage tube **= highvoltage tube

In FIG. 7, Vin=22 volts peak to peak AC signal at 1 kHz, and peak topeak voltage is measured at Vout. The input grids are biased to 22.5volts DC via R1 and R2. Resistor R3 represents an arbitrary finitesource resistance. Other R3 resistor values could have been chosen(e.g., but this may or may not have affected the maximum output voltageswing).

FIG. 8 shows a low voltage amplifier having a cascode circuit (e.g., V1Aand V1B) that has improved output swing using an output circuit similarto the circuit in FIG. 7. Here, the cathode follower uses a low voltagetube such as the 6GM8. Of course satisfactory performance in outputvoltage swing can be provided using a high voltage tube, such as forexample, a 6DJ8 as a cathode follower.

In FIG. 8, Vbias=15.6 volts DC (via Rbias), Plate voltage at pin 6 ofthe 6DJ8=18.0 volts DC. Screen grid voltage Vg2=35.7 volts DC at pin 6of V2 6BH6.

FIG. 9 shows yet another example cascode low voltage tube preamplifier,which includes the following: Vg2=30.8 volts DC, but note that whenusing high voltage tubes like the 6BH6 with low voltages, the screengrid voltage, Vg2, should be adjusted for maximum voltage swing for each6BH6 tube selected. In this case Vg2 was adjusted so that V2's quiescentplate voltage at pin 5 is 16.0 volts DC for maximum signal swing at theoutput. Also the circuit in FIG. 9 measured as follows:

Signal to noise ratio=75 dB referenced to 5 mV RMS at the input with “A”weighting with a 25 kHz single pole low pass filter and with the inputterminal shorted.

Maximum output voltage=20 volts peak to peak.

Slew rate=+13 v/μsec and −15 volts/μsec.

Harmonic distortions, HD₂=0.080% and HD₃=0.085% at 1 volt RMS output at1 kHz loaded into 10KΩ. FIG. 9 also can use conventional high voltagetubes in the cascode circuit (e.g., V1A and V1B) and or second stage(e.g., V2). In FIG. 9 the output stage cathode follower may use aconventional high voltage tube, or a low voltage tube.

FIG. 10 shows a line level preamplifier. The original design withconventional high voltage tubes was powered by a >150 volt DC supply. Itwas found that a number of tubes that will work or work “best” at 45volts. See FIG. 10. The nominal (e.g., plate) power supply voltage is 45volts and other supply voltages below 100 volts can be used. For examplea range of 16 to 35 volts for the plate power supply.

A summary of amplifier performance with examples in plate resistancesfor Rp1 and or Rp2 is presented in TABLE 6.

TABLE 6 Summary of various data on the line level preamplifier from FIG.10. Pin 1 Pin 6 Vout HD₂ HD₃ TUBE Rp1 (v) Rp2 (v) max (%) (%) 6BQ7** 127KΩ 20.5 34 KΩ 26.4 14.4 0.150 0.070 6DJ8**  87 KΩ 28.3 26 KΩ 25.6 21.00.100 0.030 6GM8*  68 KΩ 23.5 28 KΩ 18.7 26.6 0.080 0.030 6N2** 116 KΩ26.7 141 KΩ  31.2 10.3 0.200 0.120 6N6** 104 KΩ 27.6 39 KΩ 27.0 14.70.030 0.120 ECC99** 110 KΩ 26.7 141 KΩ  31.5 12.7 0.210 0.120 12AT7**123 KΩ 27.0 56 KΩ 30.0 12.4 0.240 <0.015 12AU7**  78 KΩ 24.7 43 KΩ 20.520.0 0.150 <0.015 12AX7** 123 KΩ 34.7 169 KΩ  39.4 3.1 1.200 0.60012AY7** 124 KΩ 26.5 54 KΩ 35.2 8.5 0.240 <0.015 12BH7** 112 KΩ 17.5 36KΩ 18.1 21.0 0.200 <0.015 12FQ7** 131 KΩ 19.0 32 KΩ 25.0 18.8 <0.0150.060 12U7*  67 KΩ 24.3 36 KΩ 20.5 20.0 <0.015 0.080 *= low voltage tube**= high voltage tube

TABLE 6 shows that although specially designed low voltage tubes such asthe 6GM6 (AKA ECC86) provide the most voltage swing, many conventionalhigh voltage tubes such as the 6DJ8 (AKA 6922, ECC88, or equivalent) or12AU7 and or 12BH7 provided 80% (or better) of the voltage swing outputas the 6GM6.

The closed loop gain was very close within about 10% between many of thedifferent tubes used. See TABLE 7.

TABLE 7 Closed loop gain of the amplifier in FIG. 10 with various tubes.TUBE |GAIN| 6BQ7** 9.6 6DJ8** 9.6 6GM8* 8.9 6N2** 9.9 6N6** 8.8 ECC99**9.3 12AT7** 9.7 12AU7** 8.3 12AY7** 9.6 12BH7** 9.1 12FQ7** 9.3 12U7*8.3 *= low voltage tube **= high voltage tubeFIG. 11 shows an example of a low voltage tube amplifier with cathodefollower output stage. Although in one example uses low voltage tubes,any combination of high voltage and or low voltage tubes can be used.For example: For a low voltage line preamp with maximum output voltageswing includes the following:

-   V1A V1B and V2A V2B=6GM8/ECC86,-   Rp1=86.6KΩ-   Rp2=32.4KΩ-   R4=3KΩ, 1 watt    For improvements to FIG. 11:-   C4→2.2 uf to 4.7 uf film or polystyrene capacitor. This will improve    low frequency response for those amplifier with 10KΩ input    resistance.-   R5→100Ω for lower output resistance from the preamp.-   With R5=100Ω this preamp's performance into a 10KΩ load has the    following:-   Maximum voltage output=23.5 volts peak to peak at 1 kHz-   With 1 volt RMS output at 1 kHz, HD₂=0.060%, HD₃=0.036%-   Small signal bandwidth=540 kHz.-   Also a 1 uf to 4.7 uf film or polystyrene capacitor wired in    parallel with C6 (1000 uf), ensures low impedance at high    frequencies.

FIG. 12 shows a low voltage tube headphone amplifier. The screen gridsof V1 and V2 at pin 6 are coupled nominally to a well filtered andregulated 27 volts DC. Depending on which lot of 6AU6 tubes you have,the pin 6 voltage is adjusted for maximum voltage swing at Vout withouta load resistor. Note that FIG. 11 may also be used as a line levelamplifier.

Connecting a 100Ω load to Vout yielded the following results:

Signal to noise ≥90 dB referenced to 0.50 volt RMS output (“A” weightingand 25 kHz low pass filter). At 1 volt peak to peak into 100Ω at 1 kHz,

HD₂=0.03% and HD₃=<0.03%

Small signal bandwidth=650 kHz (−3 dB)

Slew rate=+8 v/μsec, −30 v/μsec

Maximum output voltage=2.30 volts peak to peak

FIG. 13 shows an improved headphone amplifier that provides more outputcurrent into the headphone. This headphone amplifier is shown withparalleled triodes, V3 and V5 deliver sufficient volume to a headphone.The measure results are as follows:

At 2 volts peak to peak into 100Ω for 1 kHz,

HD₂=0.08% and HD₃=0.015%

With 1 volt peak to peak under the same conditions,

HD₂<0.015% and HD₃<0.015%

The circuit in FIG. 13 provided 4.5 volts peak to peak into a 100Ω loadcompared to 2.3 volts peak to peak in the previous version shown in FIG.12. With the greater signal output, the amplifier of FIG. 13 is suitableto load into a 50Ω headphone. Without a load at Vout, this amplifierdelivered 14.3 volts peak to peak, and this headphone amplifier can alsobe used as a line amplifier. Output resistor R8 can be replaced withanother value depending on the headphone. For example, if we are using250Ω to 600Ω headphones (e.g., Beyer DT 770 Pro 250 ohm or Beyer DT 990Premium 600Ω), then the output resistor R8 may be lowered to 22Ω.

To further increase gain in a single voltage amplifier stage, FIG. 14shows an example of a multiple cascode circuit. A common cathodeamplifier, V1, has its plate of V1 to a cathode of V2. Amplifier V2 isan AC grounded grid amplifier. The output plate terminal of V2 is thencoupled to the cathode of V3, another AC grounded grid amplifier. Outputis provided via plate load resistor R3 in this example from FIG. 14.

The grid bias voltages +Vbias1 and or +Vbias2 are typically a voltagevia a resistor or biasing network or voltage source. Typically, +Vbias1is less than +Vbias2 in terms of DC voltages.

As a result of having two or more AC grounded grid amplifiers (e.g.,instead on one AC grounded grid amplifier), the output resistance (e.g.,output resistance at the plate of V3 referenced to ground) is raised,which allows for higher plate load resistance at R3 to provide (e.g.,improved) higher voltage gain. FIG. 14 shows an example of two ACgrounded grid amplifiers, V2 and V3, but other implementations mayinclude more than 2 AC grounded grid amplifiers. Another embodiment mayinclude a biasing network or resistor to force “positive going” currentinto the plate of V1, V2, and or V3. By forcing extra current asmentioned, the DC plate current can be reduced and thus allow forutilizing a higher plate resistance (e.g., or impedance) load at V3 suchthat higher voltage gain is provided.

FIG. 15 shows another way to provide higher voltage gain by increasingthe effective plate resistance of V2 via an amplifier, 11.

By utilizing amplifier (e.g., in FIG. 15), 11, which for example, can beimplemented with an operational amplifier, amplifier, or for example anyamplifier shown in FIGS. 16, 17, 18, 19, and or 20. By using a feedbackamplifier wherein a non inverting input, (+), of amplifier 11, iscoupled to a bias voltage (e.g., positive voltage), Vb′ (or +Vb′), aninverting input, (−), of amplifier 11 is coupled to the cathode of V2and or plate of V1, and wherein the output of the amplifier 11 iscoupled to the grid of V2, higher voltage gain is provided at R3 via theplate of V2. The amplifier 11 also provides for higher plate resistanceat the plate of V2 (e.g., referenced to ground), which is related to thevoltage gain of amplifier 11. Amplifier 11 with a gain of greater than 0or 1, provides higher voltage gain and or output plate resistance at theplate of V2 than a conventional cascode wherein the conventional cascodecircuit would remove amplifier 11, and connect a bias voltage to thegrid of V2.

In FIG. 15 amplifier 11 may include one or more amplifying elements suchas a tube, transistor, field effect transistor, and or the like. Forexample, with a transistor, the bias voltage (e.g., Vb′ or +Vb′) iscoupled to the emitter of the transistor (e.g., the (+) input terminal),the collector (e.g., with a collector load element or load resistor)provides the output that is coupled to the grid of V2, and the cathodeof V2 is then coupled to the base of the transistor. An embodiment mayinclude a DC level shifting circuit between the collector of thetransistor and the grid of V2.

Alternatively (e.g., for amplifier 11), a field effect transistor (e.g.,FET, JFET, MOSFET) or insulated gate bipolar transistor may be usedwherein the bias voltage (e.g., Vb′ or +Vb′) is coupled to the source ofthe field effect transistor (e.g., the (+) input terminal), the drain(e.g., with a drain load element or load resistor) provides the outputthat is coupled to the grid of V2, and the cathode of V2 is then coupledto the gate of the field effect transistor or insulated gate transistor.An embodiment may include a DC level shifting circuit between the drainof the transistor and the grid of V2.

With an insulated gate transistor (e.g., for amplifier 11), the biasvoltage (e.g., Vb′ or +Vb′) is coupled to the emitter of the insulatedgate transistor (e.g., the (+) input terminal), the collector (e.g.,with a collector load element or load resistor) provides the output thatis coupled to the grid of V2, and the cathode of V2 is then coupled tothe gate of the insulated gate transistor. An embodiment may include aDC level shifting circuit between the collector of the insulated gatetransistor and the grid of V2.

An amplifier including another tube or third tube may be implemented foramplifier 11. For example, the bias voltage (e.g., Vb′ or +Vb′) iscoupled to the cathode of the tube (e.g., the (+) input terminal for thethird tube or another tube), the plate of the tube (e.g., with a plateload element or load resistor for the third tube) provides the outputthat is coupled to the grid of V2, and the cathode of V2 is then coupledto the grid of the (e.g. third) tube (e.g., the another tube or thirdtube). An embodiment may include a DC level shifting circuit between theplate of the (e.g., third) tube (the another tube or third tube) and thegrid of V2 (e.g., in FIG. 15).

FIG. 15 shows that the amplifier 11 has a non inverting input terminallabeled (+), an inverting input terminal labeled (−), and an outputterminal “A” that is coupled to the grid of tube, V2.

In FIGS. 16, 17, 18, 19, and or 20, for example, the (+), (−), and or“A” terminals or nodes correspond to the (+), (−), and or “A” terminalsor nodes in FIG. 15.

FIG. 16 shows an example (e.g., PNP) transistor circuit of amplifier 11where the non inverting input (+) is coupled to the base of Q1A, theinverting input is coupled to the base of Q2A, and the output is coupledto the collector of Q2A via a wire, or coupler circuit (e.g., 14A), or aDC level shifting circuit (e.g., 14A). A bias circuit, 12A, supplies aDC current to the emitters of Q1A and Q2A. Note that one or moreemitters from Q1A and or Q2A may include a feedback element (e.g., localand or global feedback element, or one or more emitter degenerationelements or resistors).

FIG. 17 shows an example (e.g., P-Channel) FET circuit of amplifier 11where the non inverting input (+) is coupled to the gate of Q1B, theinverting input is coupled to the gate of Q2B, and the output is coupledto the drain of Q2B via a wire, or coupler circuit (e.g., 14B), or a DClevel shifting circuit (e.g., 14B). A bias circuit, 12B, supplies a DCcurrent to the sources of Q1B and Q2B. Note that one or more sourcesfrom Q1B and or Q2B may include a feedback element (e.g., local and orglobal feedback element, or one or more source degeneration resistors orelements).

FIG. 18 shows an example (e.g., triode) tube circuit of amplifier 11where the non inverting input (+) is coupled to the grid of V1C, theinverting input is coupled to the grid of V2C, and the output is coupledto the plate of V2C via a wire, or coupler circuit (e.g., 14C), or a DClevel shifting circuit (e.g., 14C). A bias circuit, 12C, supplies a DCcurrent to the cathodes of V1C and V2C. Note that one or more cathodesfrom V1C and or V2C may include a feedback element (e.g., local and orglobal feedback element, or one or more cathode degeneration resistorsor elements). Alternatively, tubes other than triodes may be used suchas tubes including two or more grids.

FIG. 19 shows an N Channel FET implementation of amplifier 11 that issimilar to the P-Channel FET circuit in FIG. 17.

FIG. 20 shows an NPN transistor implementation of amplifier 11 that issimilar to the PNP transistor circuit in FIG. 16.

Another embodiment includes tube amplifier(s) wherein the plate and orscreen voltage operate(s) at zero volts or around zero volts (e.g.,plate or screen voltage in the range between +6 volts and −6 volts withrespect to the cathode or ground) in providing a signal output. Anexample amplifier is shown in FIG. 21A. Although FIG. 21A shows apentode, it has been found that some triodes and or multi-grid tubeswill provide signal output with plate and or screen voltages at nearzero volt or at zero volt.

The example in FIG. 21A shows a common cathode circuit to operate atnear zero or zero volt (e.g., DC voltage from plate to cathode orground), however, other configurations are included in the embodimentsuch as common grid, common plate, cascode, and or differential pair.FIG. 21A shows that the cathode of V11 is coupled to an AC ground.Coupling from cathode to ground may include a resistor, capacitor,inductor, or a circuit network (e.g., including at least one active andor passive component).

Tables 8 and 9 show summaries of some discoveries or findings (e.g.,including unexpected findings) when operating a tube circuit such as theone shown in FIG. 21A. Note that the tubes summarized in Table 8 areboth conventional high voltage types (e.g., tubes that typically arespecified for a power supply voltage of 100 volts DC or more) and alsolow voltage (e.g., automobile or space charge tubes that are specifiedat a plate voltage of 12 to 16 volts DC). Also in Tables 8 and 9 theinput signal is at 200 mV peak to peak or 200 mV peak (e.g., at 1 kHz orother frequency or frequencies). The plate load resistor (e.g., RL) hasvalues from 100KΩ to over 1MΩ. The relative gain can be calculated bythe magnitude of the output signal divided by the magnitude of the inputsignal. Also pertaining to FIG. 21A and Table 8, the cathode resistor,Rk=100Ω, and cathode capacitor, Ck was not used or connected. Of coursethe cathode of the tubes may be coupled a circuit network including aresistor, capacitor, inductor, solid state device, and or otherelectronic component(s).

In TABLES 8 and 9 other values for the load resistor may be used. Alsoother voltages including the plate supply and or grid supply and orsecond or screen grid supply may be used.

TABLE 8 Summary of various data relating to FIG. 21. Screen Plate GridVout Supply Supply pk-pk HD₂ HD₃ TUBE Brand Volts Volts Volts Notes (%)(%) 6BA6** RCA 47.0 0.0 6.150 RL < 1 MΩ — — 6BA6** RCA 6.2 0.0 2.17 RL <1 MΩ — — 6BA6** RCA 2.5 0.0 0.37 RL < 1 MΩ — — Negative peak clipping6BA6** RCA 0.87 0.0 0.061 RL < 1 MΩ — — Clean output 6BA6** RCA 0.0 0.00.006 RL < 1 MΩ — — Clean output 6BA6** RCA 0.0 0.0 0.019 RL = 1 MΩ — —Clean output 6AU6** GE #0 0.0 0.0 0.0 RL = 1 MΩ — — 6AU6** MAGNAVOX 0.00.0 0.046 RL = 1 MΩ 10.0% 3.60% 6AU6** “TRAM” 0.0 0.0 0.004 RL = 1 MΩ6AU6** GE #1 0.0 0.0 0.264 RL = 1 MΩ 30.0% 7.00% 6AU6** GE #1 0.0 Triode0.360 RL = 1 MΩ 5.0% 3.0% mode Screen grid connected to plate 6AU6** GE#1 0.0 Plate 0.470 RL = 1 MΩ — — opened Screen grid connected to plateresistor 6AU6** GE #2 0.0 0.0 0.184 RL = 1 MΩ 30.0% 10.00% 5749/ GE 0.00.0 0.070 RL = 1 MΩ 5.0% 0.30% 6BA6** 5749/ GE 0.0 2.0 0.131 RL = 1 MΩ3.0% 0.36% 6BA6** **= Conventional or high voltage tube, and — = Notmeasured.

Table 9 shows other results relating to FIG. 21A.

TABLE 9 Summary of various data relating to FIG. 21 A. Vout Screen peakPlate Grid to Supply Supply peak HD₂ HD₃ TUBE Brand Volts Volts VoltsNotes (%) (%) 6AU6** SYLVANIA 0.0 3.15 0.23 RL = 1 MΩ 2.0% 3.0% Voutocurred peaked at screen grid = 3.15, Vout was less when screen gridvoltage was <3.15 volt and slightly larger than 3.15 volts 12AC6*WESTINGHOUSE 0.0 0.0 0.0085 RL = 1 MΩ — — OR Vout slight 12DZ6*rectification, maintained correct inverted phase 12AC6* WESTINGHOUSE 0.04.12 0.0075 RL = 1 MΩ — — OR Vout has no 12DZ6* phase inversion this isunexpected 12K5* RCA 0.0 23.0 0.0045 RL << 1 MΩ 3.0% — *= low voltagetube **= high voltage tube — = not measured

From Tables 8 and 9, some of the unexpected findings or results are:

A) High voltage and or low voltage tubes operate at plate and or screengrid voltages of zero or near zero volts. Having signal output at theplate with zero or near zero supply voltage (e.g., for the plate and orscreen or second grid) is unexpected.

B) The output of the common cathode amplifier should have a normallyinverted phase referenced to the input signal's phase which was observedin all the findings except with the 12AC6 or 12DZ6 tubes (TABLE 9) whenthe plate supply voltage was about zero or zero, and when the screengrid voltage was for example at about 4.12 volts (or other voltages maybe used), which resulted in the output signal waveform at the plate tobe non-inverted (e.g., the phase of the output signal at the plate wasthe same phase as the input signal at the control grid). Having the samephase from input to output of the (e.g., common cathode) tube circuit(e.g., grid to plate) is unexpected.

C) In TABLE 9, the Sylvania 6AU6 tube exhibited more third orderharmonic distortion than second order harmonic distortion, which isunexpected (e.g., when generally the second order distortion is higherthan the third order distortion).

D) In TABLE 8, another unexpected result was stated when a GE (GeneralElectric) 6AU6 tube #1 was configured for triode mode by coupling orconnecting the screen grid to the plate and operating at near zero orzero plate supply voltage, which unexpectedly provided signal output atthe plate when the control grid was coupled to a signal generator.Another unexpected finding was that when the plate was left open orunconnected and the screen grid with near zero or zero screen gridsupply was coupled to a load (e.g., load resistor or load network), alsoa signal was outputted at the screen grid. Another unexpected result wasthat the signal outputted from the screen grid provided more signalamplitude than when the tube was triode connected with the screen gridcoupled or connected to the plate.

It should be noted that an embodiment may include when the plate wasleft open or unconnected (e.g., or uncoupled) and the screen grid withnear zero or zero screen grid supply was coupled to a load (e.g., loadresistor or load network), also signal was outputted at the screen grid,the plate may be left open or coupled to a voltage source or voltagepotential. Note the voltage source may include a signal and or DCvoltage.

In FIG. 21A it should be noted a that third grid (e.g., suppressor gridor another grid) may be coupled in any manner such as but not limited tobeing coupled to a voltage source, a ground, a cathode, another grid, ora plate. The cathode may be coupled to a signal source. Grid(s) G1 andor G2 may include a signal source and or a bias voltage.

FIG. 21B shows another embodiment wherein the operating voltage of aconventional (e.g., high voltage tube) tube and or a low voltage tubemay operate at even lower voltages by forward biasing a first grid(e.g., grid to cathode voltage ≥0 volts) and applying signal to a secondgrid and providing an output at a third grid and or at a plate.

In FIG. 21B for tube V12, a first grid G1 is coupled to bias source,+Vbias_VG1* (e.g., +Vbias_VG1*≥0) via resistor RG1. A signal inputsource is couple to G2, a second grid of V12. The third grid of V12 isshown as an example to be coupled to the cathode. The cathode of V12 maybe coupled to one or more components, or the cathode of V12 may becoupled to ground. For example, FIG. 21B shows a tube, V12, with itscathode coupled to resistor and or capacitor coupled to ground. Thecathode of V12 may be coupled to via a network such as previouslystated. A load network or resistor is coupled to the plate of V12 andthe plate provides a signal output terminal. In one embodiment, thefirst grid includes a first DC bias voltage. The cathode may be coupledto a signal source. Grid(s) G1 and or G2 may include a signal source andor a bias voltage.

For FIG. 21B, Table 10 summarized experiments with high voltage tubesthat operated with a plate supply at 12 volts or 15 volts. Note that theplate power supply is not limited to 12 volts or 15 volts, but caninclude plate supply voltages below 12 volts or other voltages. Vin=400mV pk-pk (e.g., at 1 kHz) into G2 of tube V12.

TABLE 10 Experimental results related to FIG. 21B. Vout pk-pk TUBE RG1+Vbias_VG1* RL volts NOTES 12FX5**  1 KΩ 11 VOLTS 2.4 KΩ 1.000 12FX5**100 Ω 11 VOLTS 2.4 KΩ See Vout has NOTES distorted output with lessoutput (<1.00 volt) 3S4***  12 KΩ 12 VOLTS  24 KΩ 0.800 **= high voltagetube ***= portable tube radio tube that normally operates with a platesupply ≥67.5 volts.

According to an RCA tube manual (e.g., for conventional or high voltagetubes), the second grid G2 is not a signal input grid. The first grid,G1 is not to be forward biased because that is where the AC signalsource is coupled to. The (e.g., conventional or non space charge orhigh voltage or non-automotive or non-low voltage) tubes, e.g., 12FX5and 3S4, are required that the second grid, G2 be provided with a stableor fixed DC voltage of at least 130 volts (e.g., 12FX5) and at least67.5 volts (e.g., 3S4) according to the RCA tube manual. Instead in anembodiment, in FIG. 21B the second grid is coupled to Vin, the input(e.g., AC) signal source, and the first grid is forward bias.Alternatively in another embodiment, the second grid may include the ACsignal input source plus a second DC bias voltage. It should be notedthat the first grid may include a DC bias voltage plus a second ACsignal or the first grid may include a first DC bias voltage. This biasvoltage or DC bias voltage may be ≥0 volts.

The 3S4 (e.g. 7 pin tube) and its associated family of tube such as the1R5, 1S4, 1S5, 1T4, 1L4, 1U4, 1U5, 3V4, etc. and its 8 pin counterparts,1A5, 1H5, 1N5, 1P5, 1A7, etc. have directly heated cathodes, and one ormore terminals of the filament serves as a cathode connection.

It should be noted that there are some specifically manufacturedautomotive and low voltage tubes such as the 12DS7 space charge lowvoltage tetrode that utilizes connecting the first grid (e.g., closestto the cathode) with a positive voltage (e.g., the first grid is biasedwith a positive voltage with respect to the cathode) and the second gridconnected to the input signal source. Also of note the second grid isbiased at ≤0 volt DC (e.g., with respect to the cathode) that includesan AC signal input source.

The example circuit in FIG. 21B shows a method of using conventional nonspace charge high voltage tube to be operated with low voltages byforward biasing a control grid (e.g., providing a positive voltageacross the control grid and the cathode).

The example circuit in FIG. 21B, Vin is not coupled to the first orcontrol grid G1. Vin is coupled to a second grid (e.g., a screen grid,G2, or another grid) and Vin may have an AC signal and or a DC biassignal, wherein the DC bias signal is zero, positive, or negative involtage.

For the circuit in FIG. 21B, other tubes may be used includedconventional high voltage and or low voltage (e.g., space charge orautomotive) tubes, or low voltage tubes wherein a first grid is aconventional grid that is not a spaced charge grid that is applied witha positive plate supply voltage (e.g., such as the first grid in a 12DS7space charge automobile tetrode tube that has as its first grid, a spacecharge grid, wherein the plate supply voltage is connected to it, thefirst grid).

FIG. 22A shows an example low voltage tube circuit with multiple gridsin tube V11. Although FIG. 22A shows five grid, the vacuum tube V11 mayshow N grids where N≥1. Each voltage terminals, V1, V2, V3, V4, and orV5 may include an AC signal and or a DC bias signal. The DC bias signalmay be zero, negative, or positive in voltage. The supply voltage Vs isa voltage ≥0 volt and or the voltage may be lower than the recommendedvoltage as suggested by a data sheet for that particular tube. The tubeV11 may include conventional, high voltage, non space charge,non-automotive, and or low voltage (e.g., space charge or automotive)tube type(s).

Voltage terminals, V1, V2, V3, V4, and or V5 may be provided as input oroutput terminals. For example if G1 is biased with a positive voltagevia bias circuit (e.g., a resistor or element coupled to positive DCbias voltage) and G2 is coupled to an input AC signal that may or maynot include a DC bias voltage (e.g., DC bias voltage can be zero,negative, or positive in voltage), the output signal(s) may be taken atthe plate (e.g., Vout) and or any of the other grids such as G1, G3, G4,and or G5. FIG. 22A shows a general circuit example, which allows any ofthe grids (e.g., G1, G3, G4, and or G5) may be coupled or connected toeach other, and or allows any of the grids (e.g., G1, G3, G4, and or G5)may be connected or coupled to the plate.

For example, if N=1, V11 is a triode and a lower voltage operation onthe plate supply voltage is provided by having V1 as an input signalwith a positive DC bias voltage added to an AC signal voltage.

In FIG. 22A the cathode of V11 is showed to be coupled to an AC groundor ground via resistor Rk and or capacitor Ck. Alternatively, thecathode of V11 may be coupled to another circuit or to a signal source.This circuit may include any combination of capacitor, resistor,inductor, active circuit device, circuit, another device, semiconductor,vacuum tube, amplifier, oscillator, and or the similar devices orcircuits.

In FIG. 22A example plate supply voltage Vs may be below zero, zero, orabove zero. It has been found that AC output signals were outputted whenthe supply voltage Vs is below zero, zero, or above zero. The heater orfilament of tube V11 is provided with an appropriate voltage. Note thattube V11 may include conventional high voltage and or low voltage (e.g.,space charge or automotive) tubes.

FIG. 22B shows another embodiment. Input or output terminals, Vio1,Vio2, Vio3, Vio4, and or Vio5 can be used to couple a signal into therespective grids or can be used as one or more signal outputterminal(s). Plate supply voltage Vs may be below zero, zero, or abovezero. The heater or filament of tube V11 is provided with theappropriate voltage. Again, FIG. 22B is illustrative in showing fivegrids, but in general the circuit in FIG. 22B applies to a tube with Ngrid(s) where N≥1.

A summary of example experiments is shown in TABLE 11. Note that othertubes may be used included conventional high voltage and or low voltage(e.g., space charge or automotive) tubes.

For FIG. 22B TABLE 11, Vs=+12 volts and cathode is grounded and alltubes are pentagrid tubes. However, other tubes with different number ofgrids can be used. It should be noted that different values of supplyvoltage(s), capacitor(s), and or resistors may be used. As an example,capacitors C1, C2, C3, C4, and C5 provide a low Z (or AC short circuit)or low impedance.

TABLE 11 TUBE Ra Va RL Gain NOTES 7A8** 3.0 KΩ 12 VOLTS 3.0 KΩ  >1.0 SEENOTE 1 6BE6** 3.0 KΩ 12 VOLTS 22 KΩ >1.0 SEE NOTE 2 1R5***  22 KΩ 12VOLTS 22 KΩ ~1 & >1 SEE NOTE 3 **= high voltage tube ***= portable tuberadio tube that normally operates with a plate supply ≥67.5 volts.

NOTE 1 for TABLE 11 (e.g, 7A8): A +12 volt bias voltage is coupled togrids G3 and G5 (e.g., Vc=Ve=+12 v, Rc=Re=0Ω, but other resistancevalues could be used). The AC signal is fed to Vio2 and Vio4 withVb=Vd=0 volt, and Rb=Rd=22KΩ (but other resistance values can be used).

NOTE 2 (e.g., 6BE6): Grid G5 is internally connected to the cathode andneither C5 nor Re is present in the circuit and Ve is disconnected.Grids G4 and G2 are internally connected. With equivalently, Rb=Rd=44KΩand Vb=Vd=+12 volts DC, and with input signal Vin1=Vio3 and Rc=22KΩ,|Vout/Vin1|=0.26, where Vout is at the plate or at RL. An output signalwas observed at G4 and G2, which are internally connected in this tube.The output voltage is the same at Vio2 and at Vio4, which serve asoutput terminals. The gain |Vio2/Vin1|=0.006.

In NOTE 2 with the 6BE6 tube, more gain was observed when the inputsignal was applied to grid 4 and to grid 2. The input signal is definedas Vin2=Vio2=Vio4. Gain=|Vout/Vin2|=4. Almost the same gain was observedwhen the third grid, G3 was used as an output terminal, Vio3, whichresulted in a gain=|Vio3/Vin2|=3.86. It should be noted that the controlgrid, G1, which is biased via Va=+12 volts DC and Ra=3KΩ also providedan output signal at Vio1 (e.g., this is unexpected). One aspect offorward biasing the grid or to provide a positive grid to cathodevoltage along with an input signal is that higher gain is provided ascompared to zero or negative bias between a grid and cathode.

For NOTE 3 (e.g., 1R5), grid G5 is internally connected to one of thefilament's terminal at pin 1 of the 7 pin tube. Also grids G2 and G4 areinternally connected. Effectively, Rb=Rd=44KΩ (but other resistancevalues may be used) with Vb=Vd=+12 volts DC. Also for grid, G3, Vc=+12volts DC, Rc=22KΩ. With an input signal Vin1=Vio2=Vio4,gain=|Vout/Vin1|=0.98˜1. Grid, G3 can be used as an output terminal andgain=|Vio3/Vin1|=1.34, which provides more gain than when the outputsignal was taken at the plate via Vout. Alternatively to increase signalat the plate, the input signal may be coupled to G2, G4, and G3.

Note in FIG. 22B that voltage value(s) for Va, Vb, Vc, Vd, and or Ve mayinclude other voltage values (e.g., other than +12 volts or zero volt).

FIGS. 23A, 23B, 23C, and 23D show examples of low plate supply tubecircuits. The plate supply may include zero volts, a lower thanrecommend positive voltage, near zero volts, or a slight negative platesupply voltage. An example plate supply voltage may refer to a voltagewith respect to ground or to a cathode of a tube. Although FIGS. 23A,23B, 23C, and 23D show examples of tetrodes, the tubes may includepentodes or tube with more than two grids (e.g., a three, four, five, orsix grid tube).

FIG. 23A shows a triode connected tube with the second grid for exampleis coupled to the plate.

FIG. 23B shows the second grid utilized to provide signal output viaload resistor (or a load network) wherein the plate, 101, is leftfloating, unconnected, or coupled via a resistor (or other component) toa voltage source, signal source, or ground.

FIG. 23C shows the second grid utilized to provide signal output viaload resistor (or a load network) wherein the plate, 101, is coupled toa voltage source Vp1.

FIG. 23D shows the second grid utilized to provide signal output viaload resistor (or a load network) wherein the plate, 101, is coupled toground or (e.g., alternatively) to a potential.

Note that a signal source and or bias voltage may be coupled to thecathode of any of the circuits previously mentioned including (e.g., butnot limited to) any of circuits in FIGS. 21A, 21B, 23A, 23B, 23C, and or23D.

Note that a signal source and or bias voltage may be coupled to one ormore grid(s) of any of the circuits previously mentioned including(e.g., but not limited to) any of circuits in FIGS. 21A, 21B, 23A, 23B,23C, and or 23D.

Embodiments may include the following (e.g, but not limited to):

1) A low voltage tube circuit using a vacuum tube comprising: the vacuumtube including at least a cathode or heater terminal, a plate terminal,and at least a first grid terminal, a plate power supply; a load elementwhich includes first and second terminals; coupling the cathode of thevacuum tube to ground, AC ground, or a voltage source; coupling thefirst terminal of a load element to the plate of the high voltageconventional vacuum tube, coupling a voltage source to the secondterminal of the load element; coupling a first input signal to the firstgrid or to the cathode of the vacuum tube, wherein an output signal isprovided at the plate of the vacuum tube when the plate power supply isprovided at zero volt or nearly zero volts with respect to the cathodeof the vacuum tube. The low voltage circuit may include: The low voltagetube circuit wherein the vacuum tube includes a one or more extra grids,and wherein one or more extra grids is coupled to the plate of the highvoltage conventional vacuum tube; or wherein the more extra grids iscoupled to a voltage source and or signal source which may includewherein the input signal coupled to the first grid includes an AC signaland or a DC bias signal or wherein the vacuum tube includes at least asecond grid and wherein in the second grid is supplied with a positivevoltage; or wherein the vacuum tube includes at least a second grid andwherein in the second grid is supplied with a voltage close to zerovolts; or wherein the vacuum tube is a conventional high voltage tube orwherein the vacuum tube is a low voltage vacuum tube that includes aspace charge vacuum tube or a low voltage automobile vacuum tube orwherein the second grid supplies an output signal. 2) A low voltage tubecircuit using a conventional high vacuum tube comprising theconventional high voltage vacuum tube including at least a cathode orheater terminal, a plate terminal, and first grid terminal and secondgrid terminal, a plate power supply, a load element which includes afirst terminal and a second terminal, coupling the cathode of the vacuumtube to ground, AC ground, or a voltage source, coupling the firstterminal of a load element to the plate of the high voltage conventionalvacuum tube, coupling the plate power supply to the second terminal ofthe load element, coupling a first input signal to the first grid or tothe cathode of the vacuum tube, coupling a lower than normal directcurrent voltage to the second grid, wherein lower than normal voltage isbelow the recommended voltage stated in a data sheet for theconventional high voltage vacuum tube. This low voltage tube circuit mayinclude wherein the conventional high voltage vacuum tube includes athird, fourth, fifth, or sixth grid, and wherein the input signal iscoupled to the third, fourth, fifth, or sixth grid; wherein a biasvoltage is added or coupled to the a third, fourth, fifth, or sixthgrid; or wherein the plate is decoupled from the load element or whereinthe plate is coupled to ground, and wherein the second grid provides anoutput signal; or wherein the plate power supply includes a voltage thatis lower that is recommended in the data sheet of the conventional highvoltage tube. 3) A low voltage tube circuit using a (e.g., conventionalhigh voltage) vacuum tube comprising the conventional high voltagevacuum tube comprising at least a cathode or heater terminal, a plateterminal, and first grid terminal and second grid terminal, a platepower supply, a load element which includes first and second terminals,coupling the cathode of the vacuum tube to ground, AC ground, or avoltage source, coupling the first terminal of a load element to theplate of the high voltage conventional vacuum tube, coupling the platepower supply to the second terminal of the load element, coupling aforward or positive bias voltage source to the first grid of the vacuumtube, coupling an input signal to the second or to the cathode of thevacuum tube, wherein an output signal is provided at the plate of thevacuum tube when the plate power supply is lower than a normal operatingvoltage and, wherein the forward or positive bias voltage to the firstgrid provides for operating at the lower than normal plate supplyvoltage which is below the recommended voltage stated in a data sheetfor the conventional high voltage vacuum tube. The low voltage tubecircuit may include The low voltage tube circuit may include furtherproviding an output signal at the first grid via coupling the first gridwith the bias voltage coupled to a resistor or to a circuit element, orfurther comprising a third, fourth, or Nth grid, wherein the inputsignal is coupled any combination of grids, or wherein any of the gridsmay provide one or more grid output signal or wherein any of the gridsmay include one or more grid bias voltage or, wherein any combination ofat least one grid and or cathode provides an input or output terminal,or wherein any combination of grids may include a positive or forwardbias voltage and or a signal voltage. Also it should be noted that inFIG. 21A and or 21B, capacitor(s) C5 may be coupled to the cathode andground; and for FIGS. 22A, 22B, 23A, 23B, 23C, and or 23D, capacitor(s)Ck may be coupled to the cathode and ground.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art and are intended to fallwithin the scope of the appended claims and or of the embodimentsdescribed.

That which is claimed is:
 1. A low voltage tube amplifier circuit using a vacuum tube comprising: the vacuum tube including a cathode terminal, a plate terminal, and a first grid including a first grid terminal, wherein the vacuum tube normally operates at a plate voltage of at least +12 volts; an input terminal of the low voltage tube amplifier circuit that is coupled to the first grid terminal of the vacuum tube; an output terminal of the low voltage tube amplifier circuit that is coupled to the plate terminal of the vacuum tube; a plate power supply whose voltage is at zero volt or nearly zero volts with respect to the cathode terminal of the vacuum tube; a load element which includes a first terminal and a second terminal; coupling the cathode terminal of the vacuum tube to ground; coupling the first terminal of the load element to the plate terminal of the vacuum tube; coupling the plate power supply to the second terminal of the load element; wherein coupling a first input signal to the first grid terminal of the vacuum tube provides an output signal at the plate terminal of the vacuum tube; wherein the output signal is provided at the plate terminal of the vacuum tube when the plate power supply is provided at zero volt or nearly zero volts with respect to the cathode terminal of the vacuum tube.
 2. The low voltage tube amplifier circuit of claim 1 wherein the vacuum tube includes a second grid that includes a second grid terminal.
 3. The low voltage tube amplifier circuit of claim 1 wherein the input signal coupled to the first grid terminal includes an AC signal and a DC bias signal.
 4. The low voltage tube amplifier circuit of claim 2 wherein the second grid terminal is supplied with a first positive voltage source.
 5. The low voltage tube amplifier circuit of claim 2 wherein the second grid terminal is supplied with a voltage close to zero volts.
 6. The low voltage tube amplifier circuit of claim 1 wherein the vacuum tube includes a conventional high voltage tube that normally operates at the plate voltage of over 100 volts.
 7. The low voltage tube amplifier circuit of claim 1 wherein the vacuum tube includes a space charge vacuum tube or a low voltage automobile vacuum tube.
 8. The low voltage tube amplifier circuit of claim 4 wherein the second grid terminal is coupled to a first terminal of a second load resistor and a second terminal of the second load resistor is coupled to the first positive voltage source and wherein the second grid terminal supplies a second output signal.
 9. A low voltage tube circuit using a conventional high voltage vacuum tube comprising: the conventional high voltage vacuum tube including at least a cathode or heater terminal, a plate terminal, and a first grid terminal G1 and a second grid terminal G2; a plate power supply; the conventional high voltage vacuum tube normally operates with the plate power supply whose voltage is greater than 100 volts; setting the plate power supply voltage to zero volt or near zero volts; a load element which includes a first terminal and a second terminal; coupling the cathode of the vacuum tube to ground; coupling the first terminal of the load element to the plate terminal of the high voltage conventional vacuum tube; coupling the plate power supply to the second terminal of the load element; further comprising that the low voltage tube circuit includes an input terminal that is coupled to the first grid terminal of the conventional high voltage vacuum tube; further comprising that the low voltage tube circuit includes an output terminal that is coupled to the plate terminal of the conventional high voltage vacuum tube; coupling a second grid voltage source that includes zero volt or nearly zero volts to the second grid terminal G2; wherein coupling a first signal source to the first grid terminal provides an output signal at the output terminal.
 10. The low voltage tube circuit of claim 9 wherein the conventional high voltage vacuum tube includes a third grid terminal G3, fourth grid terminal G4, or fifth grid terminal G5, and wherein the first input signal is coupled to the third grid terminal G3, or fourth grid terminal G4, or fifth grid terminal G5.
 11. The low voltage tube circuit of claim 9 wherein a bias voltage is added to the third grid terminal G3, fourth grid terminal G4, or fifth grid terminal G5.
 12. The low voltage tube circuit of claim 9 wherein the plate terminal is disconnected from the load element and wherein the second grid terminal provides an output signal.
 13. The low voltage tube circuit of claim 9 wherein the plate power supply includes a voltage of zero volt or nearly zero volts that is lower than a recommended plate terminal voltage in a data sheet of the conventional high voltage tube.
 14. A low voltage tube circuit using a vacuum tube comprising: the vacuum tube comprising at least a cathode including a cathode terminal, a plate including a plate terminal, a first grid including a first grid terminal and a second grid including a second grid terminal wherein the first grid is closest to the cathode and the second grid is further away from the first grid when the cathode is defined as a center reference; a plate power supply that includes a voltage of zero or a voltage of nearly zero volts; further comprising a load element which includes a first terminal and a second terminal; coupling the cathode terminal of the vacuum tube to a ground terminal; coupling the first terminal of the load element to the plate terminal of the vacuum tube; coupling the plate power supply to the second terminal of the load element; coupling a first positive bias voltage source to the first grid terminal of the vacuum tube; wherein an output signal is provided at the plate terminal of the vacuum tube when an input signal is coupled to the second grid terminal. 